You should consult with your carrier or with the store where you purchased your phone from and find out what your options are – whether you can get a replacement or have your unit serviced. Or if you're a DIY kind of person, feel free to check if Ifixit has a teardown of your model and if repairing it is easy. A power button spare part should cost no more than a few bucks on eBay. Just don't forget that opening your phone will void its warranty, if it has one.